Devices for unstacking flat objects are well known. A description of one such device can be found, in particular, in French patent application No. 86 10007 filed July 9, 1986 in the name of the present Applicant. Such devices are essentially constituted by a magazine for storing a stack of postal items and an unstacking head at the end of the magazine. The magazine includes drive means for presenting the first item in the stack in a well-defined position, e.g. standing upright on an edge abutting against an abutment, and with one of the sides of the item pressing against a jogging wall. The unstacking head may be constituted by a suction cup driven with a plunging motion in order to come into contact with the first item in the stack and to grasp it by suction, and driven with a translation motion in order to take said item away from its abutment position to an outlet position where it is taken by another machine located downstream. Meanwhile, the stack is advanced in the magazine and the next postal item takes the place of the preceding item.
Given the wide variety of characteristics presented by postal items, when the unstacking head moves a first, normally-grasped item to the outlet position, it is not possible to ensure that said first item is never accompanied by a second item, at least for part of the trip. It may also happen that the item grasped by the unstacking head was not in its proper position pressed against the jogging wall, but was set back from said position so that it is the second entrained item which is in fact the leading item on arrival at the outlet position. Other cases of defective un-stacking may also be considered. Overall, they give rise to a plurality of postal items being presented together at the outlet position from the unstacking head, generally in a tiled configuration, i.e. offset linearly and overlapping only partially. If these items are then driven by a conveyor towards the downstream machine, either a reject will occur, in which case unstacking will have to be performed again at a later time, or else a sorting error will occur.
The invention seeks to reduce such cases of defective unstacking and to separate items from each other when improperly presented in this way at the outlet position, insofar as it is possible to distinguish them, i.e. whenever they are sufficiently staggered.